The present invention relates to the control of multi-fuel engines and is concerned particularly, although not exclusively, with apparatus and a method for controlling a dual fuel engine, such as a diesel/gas engine.
Multi-fuel engines, i.e. engines which use more than one type of fuel, have been known for many years. One successful type of multi-fuel engine is the dual fuel engine which uses diesel oil as a base, or primary fuel, and either liquid petroleum gas (LPG), compressed natural gas (CNG) or liquefied natural gas (LNG) as an alternative or secondary fuel. Secondary fuels are often attractive because of their low cost compared to diesel oil. Some such engines, such as that described in GB 2372835 B use diesel oil and LPG as alternatives, i.e. one fuel or the other is used, with some provision for switching between the two.
Other previously considered engines, such as the example described in EP 03712309, utilise the combination of diesel and gas, which are combusted together in the engine. Combusting the two fuels together has a number of advantages when compared to the use of diesel alone. Firstly, the amount of diesel used for an equivalent engine performance can be considerably less. Additionally, a more complete combustion of the fuel can be achieved which can result in a reduction of particulates in the exhaust emission. More power, per cylinder explosion, can also be achieved.
Generally with previously considered “combination type” dual fuel engines the aim has been to reduce the amount of diesel (i.e. primary fuel) burned by the engine without losing performance. When gas (i.e. secondary fuel) is introduced into the fuel mixture burned in the cylinder the mixture burns more completely than a charge of diesel oil alone, which results in a reduction of oxygen in the exhaust gases. The reduction in oxygen content can be detected, using a lambda probe, by the engine control unit (ECU) installed by the engine manufacturer which determines that the fuel/air mixture is too rich, and accordingly reduces the amount of primary fuel which is injected.
The outcome is a reduction in primary fuel consumption, as well as lower particulates.
Thus far, dual fuel engines of the kind described above have resulted from converting single fuel engines. Conversion comes at a price, but generally it can be seen that if the cost of the secondary fuel is lower than that of the primary fuel there is the potential for economic benefit. There is also the potential for environmental benefit and performance improvements.
FIG. 1 of the accompanying drawings shows schematically an example of a previously considered control system for a dual fuel engine. This example is similar to the one described in the above-mentioned EP 03712309.
The engine, represented by 10, is a four-cylinder diesel engine, which has been converted to run on a combination of diesel and LPG. Diesel, the primary fuel, is supplied from a tank 12 by a fuel pump 14 to each cylinder into which it is injected using conventional diesel injectors. The rate of flow of the diesel fuel can be measured by a flow meter 16 and the pressure in the diesel line can be monitored by a pressure monitor 18. Air, for use in combusting the fuel, is drawn from an air intake valve 20. A gas tank 22 holds LPG for use as a secondary fuel. The gas, in liquid state, passes through a gas supply valve 24 to a vaporiser 26 where it is changed to a gaseous state before being directed to four gas injectors 28a, 28b, 28c and 28d, one for each cylinder, which are arranged to deliver the gas to a location in the air inlet manifold in a region adjacent to the air inlet valve (not shown) of each cylinder, so as to mix with the air there.
The quantity of gas delivered to the cylinders is controlled by a gas control valve 30.
An electronic controller 32 controls the gas supply valve 24 and the gas control valve 30. The controller 32 takes signals from a throttle position sensor 34, a manifold pressure sensor 36, a knock sensor 38, a crank sensor 40 (for r.p.m. information), a thermometer 42 for detecting the vaporiser temperature, and a lambda sensor 44 (for exhaust oxygen-content). An RS232 interface 46 is provided on the controller 32 to enable connection to a computer (not shown) for set up, system monitoring and diagnostics.
All of the sensors 34, 36, 38, 40, 42 and 44 are dedicated devices which are installed in the engine during the conversion process, as are the controller 32, gas injectors 28-28d, tank 22 and vaporiser 26.
Gas is supplied to the cylinders already mixed with air. The quantity of gas supplied is determined according to various factors that include the throttle position and the engine r.p.m. During the induction stroke, the gas and air mixture is drawn into the cylinder and towards the end of the compression stroke the oxygen and diesel ignite causing the gas to burn which in turn more fully and more aggressively combusts the diesel fuel, producing a greater power stroke. The combustion products which are eliminated in the exhaust stroke show reduced oxygen and particulates as compared with the combustion of diesel alone.
The above conversion can be expensive and time-consuming not least because of the number of devices which must be installed.